1. Field of the Invention
The present invention relates to a pattern inspection technique for an inspected sample having a fine circuit pattern, such as a semiconductor device and a liquid crystal display deice, particularly to method and apparatus for inspecting patterns on a wafer in the middle course of production of the semiconductor device.
2. Description of the Related Art
Examples of the inspected sample having the fine pattern include a semiconductor device, a substrate, a photomask (exposure mask), a liquid crystal display device and the like. Here, the inspection of the semiconductor wafer will be described by way of example. The semiconductor device is produced by repeating a process in which a pattern formed in a photomask is transferred onto a semiconductor wafer by lithographic treatment and etching treatment. In the semiconductor device production process, right or wrong of the lithographic treatment and etching treatment and generation of a foreign matter largely influence a yield of the semiconductor device. Accordingly, various apparatuses which inspect the pattern on the semiconductor wafer in the production process are used to early or previously detect the generation of the abnormality or defect.
For the apparatus which inspects the defect existing in the pattern on the semiconductor wafer, there is commercialized a defect inspection apparatus in which plural circuit patterns having the same shape formed on the wafer are irradiated with white light and the defect is inspected by comparing obtained optical images (for example, see Monthly Semiconductor World No. 8, 1995, pages 96 to 99). However, in the defect inspection apparatus, the defect whose size is smaller than the resolution of an optical system cannot be detected, and the defect is hardly detected in the finer and complicated circuit pattern.
Therefore, in order to inspect the circuit pattern using an electron image having a resolution higher than that of the optical image, there is proposed a method or an apparatus, wherein a secondary electron or a reflection electron generated by irradiating the semiconductor wafer with a charged particle beam such as the electron beam is detected, and the defect is detected with the use of an electron beam image based on the detected signal of the secondary electron or reflection electron. However, when the charged particle beam is used, a charging phenomenon may occur depending on the structure or material of the semiconductor wafer pattern. Unfortunately the charging phenomenon lowers image contrast or defect detection performance. Recently, because the structure or material is getting diversified with finer semiconductor wafer pattern, the defect detection performance heavily depends on a charged state of the semiconductor wafer. Therefore, the charged state of the semiconductor wafer is changed or controlled by changing or controlling energy, a current amount, the number of irradiation times of the charged particle beam or by changing or controlling capturing timing of a secondary electron signal generated from the semiconductor wafer (for example, see Japanese Patent Application Laid-Open (JP-A) No. 2005-175333).
In order to enhance defect inspection accuracy to deal with finer circuit pattern, a pixel size is reduced to improve the image resolution during the inspection. There is also disclosed a technique, wherein the pixel size is reduced only in a portion suspected to be a defect, and the inspection accuracy is improved without lengthening the inspection time (for example, see JP-A No. 2006-200944).
However, when the pixel size is simply reduced with the finer circuit pattern during the inspection, the throughput is largely degraded. For example, currently the same value of the inspected pixel size is adopted in a direction in which the charged particle beam is scanned and a direction in which the stage is moved. When the pixel size becomes a half, the inspection throughput is degraded to a quarter. When the pixel size is reduced only in the portion suspected to be the defect as described above, a process for searching the portion suspected to be the defect is required, which results in the increased inspection time.